1. Field of the Invention
The present invention relates generally to a laser source with a semiconductor laser and an optical waveguide device that are mounted on a submount.
2. Description of the Related Art
In order to achieve increases in the density of optical disks and in definition of display, a small short-wavelength light source is required. As the small short-wavelength light source, a coherent source has been attracting attention that is provided with a semiconductor laser and an optical waveguide type second harmonic generation (hereinafter, referred to as xe2x80x9cSHGxe2x80x9d) device employing a quasi-phase-matching (hereinafter, referred to as xe2x80x9cQPMxe2x80x9d) system (see Yamamoto et al., Optics Letters Vol. 16, No. 15, p. 1156, (1991)). Hereinafter, the optical waveguide type SHG device employing the QPM system is referred to as an xe2x80x9coptical waveguide type QPM-SHG devicexe2x80x9d.
FIG. 11 shows a schematic configuration of a bluish purple light source using an optical waveguide type QPM-SHG device. A wavelength-variable semiconductor laser 44 having a distributed Bragg reflection (hereinafter, referred to as xe2x80x9cDBRxe2x80x9d) region (hereinafter referred to as a xe2x80x9cDBR semiconductor laserxe2x80x9d) is used as a semiconductor laser. The DBR semiconductor laser 44 is a 100-mW class AlGaAs-based wavelength-variable DBR semiconductor laser in a 820-nm range. The DBR semiconductor laser 44 includes an active region 45, a phase adjustment region 46, and a DBR region 47. By controlling a current injected into the phase adjustment region 46 and the DBR region 47 at a certain ratio, an oscillation wavelength can be varied successively.
An optical waveguide type QPM-SHG device 48 as a wavelength conversion device includes an optical waveguide 50 and a region 51 whose polarization is reversed periodically, which are formed on an X-cut MgO-doped LiNbO3 substrate 49 as a ferroelectric substrate. The optical waveguide 50 is formed by proton exchange. The DBR semiconductor laser 44 and the optical waveguide type QPM-SHG device 48 are fixed to a submount 52 in such a manner that the surface of the DBR semiconductor laser 44 on which an active layer is formed and the surface of the optical waveguide type QPM-SHG device 48 on which the optical waveguide 50 is formed face the submount 52. A laser beam emitted from a laser beam emission portion 53 of the DBR semiconductor laser 44 is coupled directly to a laser beam entrance portion 54 of the optical waveguide 50 of the optical waveguide type QPM-SHG device 48. By carrying out an optical coupling adjustment with a laser beam being emitted from the DBR semiconductor laser 44, a 60-mW laser beam is coupled to the optical waveguide 50 with respect to a 100-mW laser output. Further, by controlling an amount of the current injected into the phase adjustment region 46 and the DBR region 47 of the DBR semiconductor laser 44, the oscillation wavelength is fixed within the allowable range of the phase matching wavelength of the optical waveguide type QPM-SHG device 48, which allows about 20-mW bluish purple light having a wavelength of 410 nm to be obtained.
Hereinafter, a configuration and a method of fabricating a laser source will be described with reference to FIG. 8.
The laser source is fabricated by disposing an optical waveguide type QPM-SHG device (optical waveguide type wavelength conversion device) 48 and a DBR semiconductor laser 44 on a submount 52. In this case, the DBR semiconductor laser 44 is fixed onto the submount 52 using solder 55 as an adhesive in such a manner that the surface of the DBR semiconductor laser 44 on which an active layer 56 is formed faces the submount 52. On the other hand, the optical waveguide type QPM-SHG device 48 is fixed onto the submount 52 with an adhesive 57 in such a manner that the surface of the optical waveguide type QPM-SHG device 48 on which the optical waveguide 50 is formed faces the submount 52. The height of the optical waveguide type QPM-SHG device 48 in the vertical direction is adjusted with spacers 58.
When fabricating this module, a device is used that mounts the optical waveguide type QPM-SHG device 48 and the DBR semiconductor laser 44 on the submount 52 by recognizing alignment markers formed on the optical waveguide type QPM-SHG device 48 and the DBR semiconductor laser 44 through image processing and positioning them using the alignment markers thus recognized. In this case, an important factor to be considered is how efficiently a laser beam emitted from the DBR semiconductor laser 44 is coupled to the optical waveguide 50. Particularly, in the short-wavelength light source including the DBR semiconductor laser 44 and the optical waveguide type QPM-SHG device 48, the power of harmonic light obtained is proportional to the square of a power of the fundamental wave to be coupled to the optical waveguide 50. Therefore, improving the optical coupling efficiency and reducing variations in the coupling efficiency among samples are particularly important.
In order to achieve high-efficiency optical coupling, it is important that the distance between a laser beam emission portion 59 of the DBR semiconductor laser 44 and a laser beam entrance portion 60 of the optical waveguide 50 of the optical waveguide type QPM-SHG device 48 is short and that the positions of the laser beam emission portion 59 and the laser beam entrance portion 60 in the horizontal direction (i.e., Y-direction) and the vertical direction (i.e., Z-direction) coincide with each other. Particularly, the position accuracy in the vertical direction is important. The optical coupling efficiency decreases significantly when there is misalignment in the vertical direction. Therefore, it is necessary that the misalignment of the laser beam entrance portion 60 of the optical waveguide 50 of the optical waveguide type QPM-SHG device 48 with respect to the emission portion 59 of the DBR semiconductor laser 44 is within xc2x10.2 xcexcm.
The distance da from the lower surface of the DBR semiconductor laser 44 to the laser beam emission portion 59 is controlled with high precision when fabricating the DBR semiconductor laser 44. Also, the position of the laser beam entrance portion 60 of the optical waveguide 50 is controlled with high precision since the optical waveguide 50 of the optical waveguide type QPM-SHG device 48 is formed on the surface of a LiNbO3 substrate 49. In addition, since the variations in size of the spacers 58 used for the position adjustment of the optical waveguide type QPM-SHG device 48 in the vertical direction are not more than xc2x10.1 xcexcm, the position adjustment of the laser beam entrance portion 60 in the vertical direction can be carried out with high precision. However, in order to achieve the position adjustment in the vertical direction with high precision so that a laser beam emitted from the DBR semiconductor laser 44 is coupled to the optical waveguide 50 with high efficiency, it is necessary to control the thickness of the solder 55 as an adhesive for fixing the DBR semiconductor laser 44.
FIGS. 9A and 9B show a conventional submount. FIG. 9A is a plan view and FIG. 9B is a front view. A submount 52 is formed using a Si substrate. On the submount 52, an electrode portion is formed that includes an electrode 61 for an active region, an electrode 62 for a phase adjustment region, an electrode 63 for a DBR region, and an electrode 64 for a ground. Films of the solder 55 as an adhesive portion for fixing a DBR semiconductor laser are formed on the electrodes 61 to 63 (not on the electrode 64 for a ground), respectively. Alignment markers 65 used for the position adjustment of a DBR semiconductor laser and an optical waveguide device fixing portion 66 further are formed on the submount 52. The electrode 61 for an active region, the electrode 62 for a phase adjustment region, the electrode 63 for a DBR region, the electrode 64 for a ground, the alignment markers 65, and the optical waveguide device fixing portion 66 are formed by sputtering and have the same thickness (0.5 xcexcm). The thickness of the films of the solder 55 is 3 xcexcm.
Next, a conventional method for fixing a DBR semiconductor laser to a submount will be described with reference to FIGS. 10A and 10B. First, as shown in FIG. 10A, a DBR semiconductor laser 44 is brought above a solder 55 following alignment markers 65. Next, as shown in FIG. 10B, the DBR semiconductor laser 44 is placed on the solder 55 while applying a load thereto. The submount 52 is heated by a heater to melt the solder 55 and then cooled. The DBR semiconductor laser 44 thus is fixed on the submount 52. The position control of the laser beam emission portion 59 of the DBR semiconductor laser 44 in the height direction conventionally has been carried out by controlling the load applied to the DBR semiconductor laser 44 when fixing the DBR semiconductor laser 44, the temperature for melting the solder 55, and the thickness of the films of the solder 55 as an adhesive portion for fixing the DBR semiconductor laser 44. However, after the DBR semiconductor laser 44 has been fixed, the films of the solder 55 have a thickness of 2xc2x10.5 xcexcm. That is, the thickness of the films of the solder 55 shows variations of xc2x10.5 xcexcm. This causes the position of the laser beam emission portion 59 of the DBR semiconductor laser 44 in the height direction to vary xc2x10.5 xcexcm, which makes it difficult to achieve high-efficiency optical coupling stably.
Thus, in order to achieve high-efficiency optical coupling stably, the mounting accuracy needs to be improved further. In addition, the DBR semiconductor laser 44 may be mounted in an inclined manner so that the contact area between the DBR semiconductor laser 44 and the films of the solder 55 as an adhesive portion varies among samples, which may result in variations in characteristics of the DBR semiconductor laser 44 among samples and a shortened lifetime of the DBR semiconductor laser 44 due to the degraded heat dissipation. Moreover, when mounting the DBR semiconductor laser 44 on the submount 52, the DBR semiconductor laser 44 may warp, which may result in variation in characteristics of the DBR semiconductor laser 44 among samples and a degraded reliability of the lifetime of the DBR semiconductor lasers 44. Therefore, in view of the above-mentioned problems, achieving high-efficiency optical coupling and improving the characteristics of the DBR semiconductor laser 44 have been desired.
In order to solve the above-mentioned problems, a laser source according to the present invention includes a submount, an electrode portion formed on the submount, and a semiconductor laser fixed to the electrode portion by means of an adhesive portion, wherein the submount comprises a stopper portion, and the semiconductor laser is fixed to the electrode portion by means of the adhesive portion while being in contact with the stopper portion.
In the above-mentioned laser source according to the present invention, it is preferable that relationships of d1 less than d2 and d1+d3≅d2 are satisfied, where d1 denotes a thickness of the electrode portion, d2 denotes a thickness of the stopper portion as measured from an upper surface of the submount, and d3 denotes a thickness of the adhesive portion.
Further, in the above-mentioned laser source according to the present invention, it is preferable that the submount comprises a plurality of stopper portions.
Furthermore, in the above-mentioned laser source according to the present invention, it is preferable that an adhesive portion having a predetermined width is formed along an optical waveguide portion of the semiconductor laser, pairs of stopper portions face each other with the adhesive portion intervening therebetween, and a distance between the stopper portions facing each other is greater than the predetermined width of the adhesive portion and smaller than a width of the semiconductor laser.
Still further, in the above-mentioned laser source according to the present invention, it is preferable that the stopper portion is formed at a position apart from the electrode portion.
Still further, in the above-mentioned laser source according to the present invention, it is preferable that a groove is formed between the stopper portion and the electrode portion.
Still further, in the above-mentioned laser source according to the present invention, it is preferable that the semiconductor laser and the electrode portion are electrically contacted with each other via the adhesive portion.
Still further, in the above-mentioned laser source according to the present invention, it is preferable that the laser source further includes an optical waveguide device, which is disposed on the submount.
In this case, it is preferable that an optical waveguide device fixing portion for fixing the optical waveguide device is formed on the submount, and a thickness of the optical waveguide device fixing portion is substantially the same as that of the stopper portion as measured from an upper surface of the submount. In this case, it is preferable that the stopper portion and the optical waveguide device fixing portion are formed at the same time by the same process. Also, in this case, it is preferable that the stopper portion and the optical waveguide device fixing portion are formed by sputtering or plating. Also, in this case, it is preferable that a surface material of the optical waveguide device fixing portion is at least one material selected from the group consisting of Al, Cr, Ta, Ti, Si, Cu, Mo, and W. Also, in this case, it is preferable that the stopper portion and the optical waveguide device fixing portion are formed by etching.
Still further, in this case, it is preferable that the optical waveguide device is fixed to the submount with an ultraviolet curable resin via a spacer.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.